An estimated 30 million American men are reported to have some form of erectile dysfunction with close to 75 percent having an organic origin. The erectile response in the penis is dependent on the reactive state of the smooth muscle of the cavernosal sinuses and of the arteries supplying blood to those sinuses. A variety of pathways can sensitize the cavernosal vasculature and suppress the normal erectile response. Our preliminary studies, using an in vivo model of erectile function have demonstrated both ET-1 and pheneylphrine can suppress the erectile response and that an inhibitor of Rho-kinase is capable of reversing this sensitization. We hypothesize that an active RhoA/Rho-kinase signal transduction pathway exerts a potent vasoconstrictive action on the cavernosal smooth muscle thus maintaining the penis in a flaccid state. Activation of the RhoA pathway results in increased activity of Rho-associated kinase (ROK or Rho-kinase) which enhances contractile sensitivity. The goal of this proposal is to understand the role of the RhoA/Rho-kinase signaling vasoactive pathways and mechanisms by which Rho-kinase modulates cavernosal smooth muscle contractile activity. Specific aim 1 is to demonstrate the functional role of the RhoA/Rho-kinase signal transduction pathway in normal erectile response. Specific aim 2 is to elucidate how Rho-kinase activated cavernosal smooth muscle contraction influences the erectile response mediated by neural stimulation or pharmacological intervention. Specific aim 3 is to determine if the RhoA/Rho-kinase pathway contributes to the diminished erectile response in an animal model of erectile dysfunction. With use of both in vivo and in vitro approaches, this proposal will generate new information relevant to the regulation of cavernosal circulation and its role in erectile dysfunction. If our hypothesis is correct, future studies may examine other modulators of this novel regulatory pathway and how potential alterations of this pathway may aid in treatment of erectile dysfunction.